The planning fallacy is related to optimism bias (think Lake Wobegon—where all the children are above average) and self-serving bias (where the good stuff is deemed to be my doing while the bad stuff is always someone else’s fault). We routinely overrate our own capacities and exaggerate our abilities to shape the future.
Thus the planning fallacy is our tendency to underestimate the time, costs and risks of future actions and at the same time to overestimate the benefits thereof. It’s at least partly why we underestimate the likelihood of bad results. It’s why we think it won’t take us as long to accomplish something as it does. It’s why projects tend to cost more than we expect. It’s why the results we achieve aren’t nearly as good as we expect and why they are so often disastrous.
More here – Think Advisor